Typically, a barrel and feedscrew are used to mix and melt various materials for extrusion, blowmolding, and injection processes. For example, bulk plastic material is fed into an extruder, an injection molding machine, or a blow molder through a barrel or cylinder utilizing a rotating helical screw. Such a machine is operated at an elevated temperature and if it is required to be shut down, the plastic material tends to solidify. Many plastic materials are highly abrasive and tend to wear the flights on the screw thereby widening the gap between the outer edge of the flights and the inner surface of the barrel. Such a condition will tend to prevent the uniform free flow of material through the barrel thereby causing problems in the consistency of the plastic material. Other problems that can occur are galling in the barrel through adhesive wear, metal to metal contact within the barrel, and misalignment of the screw through excessive shear or deflection of the screw mechanism.
Previously, the only way to check the dimensional clearances in such a mechanism was to shut it down and remove the screw from the barrel. Since the plastic material would solidify, the screw and the barrel would have to be cleaned and the dimensions checked utilizing mechanical measuring devices. Then, the mechanism would have to be reassembled. Such a procedure tended to result in lost production time and did not completely eliminate the occasional breakdowns between regular measurements.
In U.S. Pat. No. 4,604,251, there is shown an apparatus and method for checking the dimensional relationship between the screw flights and the inner diameter of a barrel during the operation of the plastic material feed device. Utilizing the Foucault current or eddy method of detection, a probe is located in an aperture formed in the side of the barrel. The end of the probe includes a sensor positioned near the inner surface of the barrel. The probe generates an electrical signal having a magnitude proportional to the distance between the outer edge of the flight on the screw and a sensing coil located in the sensor of the probe. The probe output signal is sensed and converted to a digital distance display for use by the machine operator. The magnitude of the signal can be scaled to generate a display number representing the actual measurement between the edge of the flight screw and the inside of the barrel in English or metric units. The signal can be stored, selectively reset, and two or more probe detection signals can be selectively displayed.
The most time efficient manner of measuring the wear on the screw involves measuring the relevant distances within the barrel during rotation of the screw therein. However, one problem encountered when attempting to measure the distances within the barrel using the aforementioned probe relates to the heat generated by the rotation of the screw within the barrel. The heat generated by use of the screw can potentially damage the sensor of the probe if the sensor is exposed to elevated temperatures within the barrel for an extended period of time. There accordingly exists a need for a method of quickly installing the probe and making the proper measurements prior to the probe undergoing damage as a result of exposure to heat within the barrel.
In U.S. Pat. No. 7,168,291, there is further shown an apparatus and method for checking the amount of wear experienced by the inner surface of the barrel or cylinder in addition to the wear experienced by the flight of the screw. A plug having an inner contour matching that of the inner surface of the barrel or cylinder is placed into the aperture formed in the side of the barrel or cylinder and retained therein by a plug retainer. The plug remains in position during an extended period of use of the screw to cause the inner contour of the plug to undergo wear in a manner consistent with the remainder of the inner surface of the barrel or cylinder. The plug is then removed from the aperture and the degree of wear experienced by the plug is determined by measuring an axial length of the plug and comparing the measured length to the length of the plug prior to placement in the aperture of the barrel or cylinder.
The measurement of the axial length of the plug may require the use of additional measuring devices and fixtures following the removal of the plug from the barrel, thereby further complicating the process of determining the wear experienced by the barrel.
There accordingly exists a need for a method and apparatus suitable for measuring both the wear experienced by the screw and the wear experienced by the barrel or cylinder accommodating the screw in an accurate and time efficient manner while utilizing a reduced number of components. There further exists a need to utilize these measured and recorded values in order to predictively maintain a production installation having the barrel and feedscrew assembly by preventatively avoiding undesirable part damage or failure.